Advice/Recommendations for Traveling around Europe starting from Switzerland

Hello! Back in 2009 I became close friends with a Foreign Exchange student from Switzerland at my High School in Missouri. When she returned home, that following year I had the awesome background to visit her and spend an entire month in Switzerland with her family right before starting college. Back in 2012 we figured out we would *roughly be finishing our Undergrad programs at about the same time, and decided to keep the idea open of a big trip together again when that final year rolled around, and this is it! (She graduates in July, Ill finish my Capstone this Fall September-December). The last trip, we went all over Switzerland, but never crossed the border and to date that is my only trip out of the U.S. so far. The current plan is to fly out of Missouri about July 24th for Zurich, and then return ~August 15th. I will fly into Zurich, and then the plan is to hit the road! (This go around, my friend has her own car we can take to wherever we desire). Im pretty open-ended about where we go and what we do, but she made the request we try to do more "nature" oriented stuff than stick around cities too much after she just returned from a year in Australia. Ive only been to Switzerland, so I am excited to get out and see everything. One thought we had was to head South to Italy and then work our way counter-clockwise bearing Westward. Given the nature request and that we both enjoy hiking, one thought was to head East towards Croatia (an area I have never given much thought to, but looks beautiful for the forests). It has been several years, and I know that can change a lot, so I was wondering if anyone had particular places of interest to recommend checking out? I know this is kind of open-ended, but all ideas are welcome! Its been 6 years since I was in that far abroad and may be another before I have the chance to return. For traveling purposes, we are trying to have fun but be frugal on necessary costs as possible. As such, the plan is to hit up several hostels and camp when the weather is nice. Also, I wondered if some of the more well-seasoned travelers here could recommend how to handle the money situation being abroad for so long? The last time, I bought the plane ticket in US, and then went to an Exchange in Switzerland where I got about $500 US worth of Swiss Franks for my stay (which lasted me quite well since her family covered most other expenses).